What does the company do well? We moved our sites to WP engine for two reasons. One was reliability and the other was upgrades. Switching to a managed hosting provider that notifies you of upgrades and then does them was a lifesaver. We were constantly having issues with our sites being hacked and not being up to date but when we switched to WP Engine we were able to save money on the security plugins we were using and the site is more secure and faster due to the built in caching.
What does the company not do well? The back end is quite complex and getting sites switched over to them was a bit of a pain. We use a lot of subdomains on our sites and we were constantly having issues getting everything setup on multisite installs. So if you are moving a number of sites over be prepared for a few hassles.
What do you wish you had known about the company before using? The biggest thing I wish I had done before we moved out sites over is read up on the process and tested it out with smaller sites. This would have saved us quite a bit of downtime on our sites.